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          <h2 class="post-title" itemprop="name headline">如何利用redis实现缓存</h2>
        

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        <p>redis是典型的非关系型数据库，支持key-value，hash，list，set等各种数据结构。那么如何利用redis实现缓存呢？</p>
<a id="more"></a>

<h2 id="接口定义"><a href="#接口定义" class="headerlink" title="接口定义"></a>接口定义</h2><p>首先，我们需要定义一个数据包装类，用来包装缓存的值，为什么需要包装类呢？举个例子，假设我们缓存了一个null值，那么缓存返回的也是null值，使用者怎么知道这个null是说缓存中没数据还是缓存的就是null？</p>
<p>CacheWrapper类:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">CacheWrapper</span>&lt;<span class="title">E</span>&gt; <span class="keyword">implements</span> <span class="title">Serializable</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> E value;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">boolean</span> exist = <span class="keyword">false</span>;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">CacheWrapper</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">CacheWrapper</span><span class="params">(E e)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.value = e;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">CacheWrapper</span><span class="params">(E value, <span class="keyword">boolean</span> exist)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.value = value;</span><br><span class="line">        <span class="keyword">this</span>.exist = exist;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> E <span class="title">getValue</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> value;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setValue</span><span class="params">(E value)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.value = value;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">isExist</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> exist;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setExist</span><span class="params">(<span class="keyword">boolean</span> exist)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.exist = exist;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">equals</span><span class="params">(Object o)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">if</span> (<span class="keyword">this</span> == o) <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">        <span class="keyword">if</span> (!(o <span class="keyword">instanceof</span> CacheWrapper)) <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line"></span><br><span class="line">        CacheWrapper&lt;?&gt; that = (CacheWrapper&lt;?&gt;) o;</span><br><span class="line"></span><br><span class="line">        <span class="keyword">if</span> (isExist() != that.isExist()) <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line">        <span class="keyword">return</span> getValue() != <span class="keyword">null</span> ? getValue().equals(that.getValue()) : that.getValue() == <span class="keyword">null</span>;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">hashCode</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">int</span> result = getValue() != <span class="keyword">null</span> ? getValue().hashCode() : <span class="number">0</span>;</span><br><span class="line">        result = <span class="number">31</span> * result + (isExist() ? <span class="number">1</span> : <span class="number">0</span>);</span><br><span class="line">        <span class="keyword">return</span> result;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>接下来，我们需要定一个接口，方便未来扩展。</p>
<p>Cache接口:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Cache</span>&lt;<span class="title">K</span>, <span class="title">V</span>&gt; </span>&#123;</span><br><span class="line">    <span class="comment">// 设置</span></span><br><span class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">put</span><span class="params">(K k, V v)</span></span>;</span><br><span class="line">    <span class="comment">// 获取</span></span><br><span class="line">    <span class="function">CacheWrapper&lt;V&gt; <span class="title">get</span><span class="params">(K k)</span></span>;</span><br><span class="line">    <span class="comment">// 清除指定key</span></span><br><span class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">expulse</span><span class="params">(K k)</span></span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="key-value序列化"><a href="#key-value序列化" class="headerlink" title="key-value序列化"></a>key-value序列化</h2><p>我们知道，redis只能存string/byte数组/int/long等基础类型的数据，一般我们用的比较多的也是string类型。那么针对java中众多对象，我们需要定义一个序列化方法和反序列化方法，方便存取数据。</p>
<p>简单来说，使用json序列化和反序列化可以满足需求。但是json序列化得到的数据长度较长，占内存多。所以我们选择了hessian。</p>
<p>以下是HessianSerialize静态类:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> com.caucho.hessian.io.HessianInput;</span><br><span class="line"><span class="keyword">import</span> com.caucho.hessian.io.HessianOutput;</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> java.io.ByteArrayInputStream;</span><br><span class="line"><span class="keyword">import</span> java.io.ByteArrayOutputStream;</span><br><span class="line"><span class="keyword">import</span> java.io.IOException;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HessianSerialize</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">byte</span>[] _serialize(Object obj) <span class="keyword">throws</span> IOException &#123;</span><br><span class="line">        <span class="keyword">if</span> (obj == <span class="keyword">null</span>) <span class="keyword">throw</span> <span class="keyword">new</span> NullPointerException();</span><br><span class="line">        ByteArrayOutputStream os = <span class="keyword">new</span> ByteArrayOutputStream();</span><br><span class="line">        HessianOutput ho = <span class="keyword">new</span> HessianOutput(os);</span><br><span class="line">        ho.writeObject(obj);</span><br><span class="line">        <span class="keyword">return</span> os.toByteArray();</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="keyword">static</span> Object <span class="title">_deserialize</span><span class="params">(<span class="keyword">byte</span>[] by)</span> <span class="keyword">throws</span> IOException </span>&#123;</span><br><span class="line">        <span class="keyword">if</span> (by == <span class="keyword">null</span>) <span class="keyword">throw</span> <span class="keyword">new</span> NullPointerException();</span><br><span class="line">        ByteArrayInputStream is = <span class="keyword">new</span> ByteArrayInputStream(by);</span><br><span class="line">        HessianInput hi = <span class="keyword">new</span> HessianInput(is);</span><br><span class="line">        <span class="keyword">return</span> hi.readObject();</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">byte</span>[] serialize(Object obj) <span class="keyword">throws</span> Exception &#123;</span><br><span class="line">        <span class="keyword">if</span> (obj == <span class="keyword">null</span>) <span class="keyword">throw</span> <span class="keyword">new</span> NullPointerException();</span><br><span class="line">        <span class="keyword">return</span> _serialize(obj);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Object <span class="title">deserialize</span><span class="params">(<span class="keyword">byte</span>[] by)</span> <span class="keyword">throws</span> Exception </span>&#123;</span><br><span class="line">        <span class="keyword">if</span> (by == <span class="keyword">null</span>) <span class="keyword">throw</span> <span class="keyword">new</span> NullPointerException();</span><br><span class="line">        <span class="keyword">return</span> _deserialize(by);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="redisCache实现类"><a href="#redisCache实现类" class="headerlink" title="redisCache实现类"></a>redisCache实现类</h2><p>接下来就是实现代码了，在实现中需要配置一下expireTime，防止数据无限缓存，还有在出现异常时，是否需要抛出异常。</p>
<p>RedisCache类：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br><span class="line">100</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> redis.clients.jedis.Jedis;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">RedisCache</span>&lt;<span class="title">K</span>, <span class="title">V</span>&gt; <span class="keyword">implements</span> <span class="title">Cache</span>&lt;<span class="title">K</span>, <span class="title">V</span>&gt; </span>&#123;</span><br><span class="line">    <span class="comment">// 默认的缓存时间，如果不设置，数据一直保存在redis中，对redis来说压力太大</span></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> DEFAULT_CACHE_TIME_LIMIT = <span class="number">24</span> * <span class="number">60</span> * <span class="number">60</span>;</span><br><span class="line">    <span class="comment">// 出现异常是否静默</span></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">boolean</span> throwSilent = <span class="keyword">true</span>;</span><br><span class="line">    <span class="comment">// 缓存时间</span></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> defaultCacheTimeLimit = DEFAULT_CACHE_TIME_LIMIT;</span><br><span class="line">    <span class="comment">// redis</span></span><br><span class="line">    <span class="keyword">private</span> Jedis jedis;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 先set已经序列化的数据，value值被CacheWrapper包装</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> k</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> v</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span></span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">put</span><span class="params">(K k, V v)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            jedis.set(HessianSerialize.serialize(k), HessianSerialize.serialize(<span class="keyword">new</span> CacheWrapper&lt;V&gt;(v, <span class="keyword">true</span>)));</span><br><span class="line">            jedis.expire(SerializeUtil.hessian2Serialize(k), defaultCacheTimeLimit);</span><br><span class="line">            <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">        &#125; <span class="keyword">catch</span> (Exception e) &#123;</span><br><span class="line">            <span class="keyword">if</span> (throwSilent) &#123;</span><br><span class="line">                e.printStackTrace();</span><br><span class="line">                <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line">            &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                <span class="keyword">throw</span> <span class="keyword">new</span> RuntimeException(e);</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 获取数据</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> k</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span></span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> CacheWrapper&lt;V&gt; <span class="title">get</span><span class="params">(K k)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            <span class="keyword">byte</span>[] result = jedis.get(HessianSerialize.serialize(k));</span><br><span class="line">            <span class="keyword">if</span> (result == <span class="keyword">null</span>) &#123;</span><br><span class="line">                <span class="keyword">return</span> <span class="keyword">new</span> CacheWrapper&lt;V&gt;(<span class="keyword">null</span>, <span class="keyword">false</span>);</span><br><span class="line">            &#125;</span><br><span class="line">            <span class="keyword">return</span> (CacheWrapper&lt;V&gt;) HessianSerialize.deserialize(result);</span><br><span class="line">        &#125; <span class="keyword">catch</span> (Exception e) &#123;</span><br><span class="line">            <span class="keyword">if</span> (throwSilent) &#123;</span><br><span class="line">                e.printStackTrace();</span><br><span class="line">                <span class="keyword">return</span> <span class="keyword">new</span> CacheWrapper&lt;V&gt;(<span class="keyword">null</span>, <span class="keyword">false</span>);</span><br><span class="line">            &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                <span class="keyword">throw</span> <span class="keyword">new</span> RuntimeException(e);</span><br><span class="line">            &#125;</span><br><span class="line"></span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 清除缓存</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> k</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span></span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">expulse</span><span class="params">(K k)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            jedis.del(HessianSerialize.serialize(k));</span><br><span class="line">            <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line">        &#125; <span class="keyword">catch</span> (Exception e) &#123;</span><br><span class="line">            <span class="keyword">if</span> (throwSilent) &#123;</span><br><span class="line">                e.printStackTrace();</span><br><span class="line">                <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line">            &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                <span class="keyword">throw</span> <span class="keyword">new</span> RuntimeException(e);</span><br><span class="line">            &#125;</span><br><span class="line"></span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">isThrowSilent</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> throwSilent;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setThrowSilent</span><span class="params">(<span class="keyword">boolean</span> throwSilent)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.throwSilent = throwSilent;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">getDefaultCacheTimeLimit</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> defaultCacheTimeLimit;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setDefaultCacheTimeLimit</span><span class="params">(<span class="keyword">int</span> defaultCacheTimeLimit)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.defaultCacheTimeLimit = defaultCacheTimeLimit;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Jedis <span class="title">getJedis</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> jedis;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setJedis</span><span class="params">(Jedis jedis)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.jedis = jedis;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="缓存过期策略"><a href="#缓存过期策略" class="headerlink" title="缓存过期策略"></a>缓存过期策略</h2><p>缓存过期策略是指由于缓存大小有限，当新的缓存数据加入进来的时候，需要清理掉旧的缓存数据，腾出有限空间。</p>
<p>缓存过期策略分为以下三种：</p>
<ul>
<li><strong>FIFO</strong>：First In First Out，先进先出</li>
<li><strong>LRU</strong>：Least Recently Used，最近最少使用</li>
<li><strong>LFU</strong>：Least Frequently Used，最不经常使用</li>
</ul>
<p>原理和实现如下：</p>
<h3 id="FIFO"><a href="#FIFO" class="headerlink" title="FIFO"></a>FIFO</h3><p>FIFO按照“先进先出（First In，First Out）”的原理淘汰数据，正好符合队列的特性，数据结构上使用队列Queue来实现。<br>如下图：</p>
<p><img src="/images/pasted-103.png" alt="upload successful"></p>
<ol>
<li><p>新访问的数据插入FIFO队列尾部，数据在FIFO队列中顺序移动；</p>
</li>
<li><p>淘汰FIFO队列头部的数据；</p>
</li>
</ol>
<h3 id="LRU"><a href="#LRU" class="headerlink" title="LRU"></a>LRU</h3><p>LRU（Least recently used，最近最少使用）算法根据数据的历史访问记录来进行淘汰数据，其核心思想是“如果数据最近被访问过，那么将来被访问的几率也更高”。</p>
<p>最常见的实现是使用一个链表保存缓存数据，详细算法实现如下：</p>
<p><img src="/images/pasted-104.png" alt="upload successful"></p>
<ol>
<li><p>新数据插入到链表头部；</p>
</li>
<li><p>每当缓存命中（即缓存数据被访问），则将数据移到链表头部；</p>
</li>
<li><p>当链表满的时候，将链表尾部的数据丢弃。</p>
</li>
</ol>
<h3 id="LFU"><a href="#LFU" class="headerlink" title="LFU"></a>LFU</h3><p>LFU（Least Frequently Used）算法根据数据的历史访问频率来淘汰数据，其核心思想是“如果数据过去被访问多次，那么将来被访问的频率也更高”。</p>
<p>LFU的每个数据块都有一个引用计数，所有数据块按照引用计数排序，具有相同引用计数的数据块则按照时间排序。</p>
<p>具体实现如下：</p>
<p><img src="/images/pasted-105.png" alt="upload successful"></p>
<ol>
<li><p>新加入数据插入到队列尾部（因为引用计数为1）；</p>
</li>
<li><p>队列中的数据被访问后，引用计数增加，队列重新排序；</p>
</li>
<li><p>当需要淘汰数据时，将已经排序的列表最后的数据块删除。</p>
</li>
</ol>
<h3 id="redis中的实现"><a href="#redis中的实现" class="headerlink" title="redis中的实现"></a>redis中的实现</h3><p>操作redis数据不像操作java集合一样方便，如果redis内存足够大，我们可以模拟以上三种过期策略。</p>
<p>在本文代码中，我们统一设置了缓存失效时间，也就是说先缓存的数据会先被清理掉，这和FIFO策略很类似。</p>
<p>如何实现LRU呢？我们可以在get数据时，如果在redis中得到了key和对应的value，就刷新key的过期时间expireTime，这就相当于将最近使用的key放到了链表的表头。</p>
<p>如何实现LFU？LFU比LRU高级一点，需要对每个key的get次数计数，这种redis操作也比较难，那如何实现呢？我们可以在get到数据后，在这个key的过期时间上再加一个countTime计数时间。相当于每get一次key，这个key的过期时间就会被增加一点，变相实现了LFU。</p>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>为什么会想用redis做缓存？之前调研过Ehcache和Guava中的Cache，参见<a href="/2016/11/04/ehcache-and-guava-cache/">Ehcache与Guava Cache之间的区别</a>，<a href="/2016/10/27/guava-cache-study/">Guava学习：Cache缓存入门</a>。Ehcache虽然使用RMI实现了分布式缓存，但使用起来配置较多，较复杂，Guava Cache虽简单易用，但是仅限于单jvm使用。</p>
<p>redis经过简单的封装就能给跨jvm应用提供缓存。</p>

      
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    <strong>本文作者：</strong>
    Gavin Zhang
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    <strong>本文链接：</strong>
    <a href="http://blog.gavinzh.com/2018/08/06/how-to-use-cache-in-redis/" title="如何利用redis实现缓存">http://blog.gavinzh.com/2018/08/06/how-to-use-cache-in-redis/</a>
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    <strong>版权声明： </strong>
    本博客所有文章除特别声明外，均采用 <a href="https://creativecommons.org/licenses/by-nc-sa/3.0/" rel="external nofollow" target="_blank">CC BY-NC-SA 3.0</a> 许可协议。转载请注明出处！
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            <a href="/tags/java/" rel="tag"># java</a>
          
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            <a href="/tags/Ehcache/" rel="tag"># Ehcache</a>
          
            <a href="/tags/Cache/" rel="tag"># Cache</a>
          
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